The present invention relates to a disk grinder or the like, and more particularly, to the grinding plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the wood fibers are worked between two refiner plates which usually have radial bars and grooves. A large volume of steam is produced between the plates as a result of this refining work. For effective refining, the fibers must be retained between the plates on the bar surfaces despite the high velocity of the flowing steam, and the enormous centrifugal forces. Typically, dams are provided in the grooves, to interrupt material flow and thus improve the retention time of the material in the refining region.
In a typical refiner plate with radial bars and grooves, the bars provide impacts or pressure pulses which separate and fibrillate the fibers. The grooves enable feeding and steam extraction. Near the perimeter, a high radial steam flow and centrifugal force both act to sweep the fibers from between the plates, thus reducing the refining effectiveness. The flow restrictions of a small plate gap and fiber-filled grooves result in a steam pressure peak between the plates. This pressure peak is a major portion of the refining thrust, and contributes to control instability at high motor loads. Dams in the grooves help to retain the fibers and force them to the bar surface. However, they also further restrict the steam flow.
It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while retaining the pulp within the region as long as possible. U.S. Pat. No. 4,676,440 discloses a grinding plate pattern which includes exhaust channels to assist in the removal of steam from the refining region. The patent more specifically discloses two oppositely placed grinding disc plates, each made up of several segments attached to each other, at least one of the discs being rotatable. The segments are provided with a rough surface or with bars and grooves for the grinding of material such as wood chips. The segments of at least one of the plates are provided with one or more exhaust channels having a sectional area essentially larger than that of the grooves. The exhaust channels are superimposed on, and span, the outer two of three refining zones arranged on the plate in the direction of material passage. One significant disadvantage arises from the fact that the fabrication of the plate surface of the type disclosed in U.S. Pat. No. 4,676,440, is quite costly.